I guess that means 80% are at anchor or tied to the slip when they sink.
I have a buddy who installed a 120V sump pump attached to shore power. In the event it is activated, by high water, an obnoxious buzzer goes off during the entire time the pump is running.
It is a powerful backup knowing you have a pump aboard that will not die if and when the battery gives way.
The pump is located where it can be pulled quickly and given to anyone who needs it in an emergency. Not approved marine type equipment, but it could keep your boat from sinking.
Here is an interesting side bar from Bilge Monitor, selling their alarm system.
http://www.bilgemonitor.com/
Paul